The heat generation amount of a power element for supplying electric power is comparatively high among semiconductor elements. As a substrate which mounts the power element, as shown in Patent Document 1 for example, there is used a power module substrate such that an Al (aluminum) metal plate is bonded onto a ceramic substrate composed of AlN (aluminum nitride) via a brazing filler material.
Moreover, the metal plate serves as a circuit layer, and over the metal plate, there is mounted a power element (semiconductor element) via a soldering material.
Also, there is proposed a power module substrate having a heat sink such that a metal plate made of Al or the like is bonded on the underside surface of a ceramic substrate, thereby forming a metal layer, and a heat sink is bonded on the metal layer.
Moreover, as a means of forming a circuit layer, there is a method in which a metal plate is bonded onto a ceramic substrate, and then a circuit pattern is formed on the metal plate. Furthermore, as disclosed in Patent Document 2 for example, there is a method in which a metal piece which is preliminarily formed in a circuit pattern shape is bonded onto a ceramic substrate.
Here, in order to obtain superior bonding strength between the metal plate and the ceramic substrate, for example, Patent Document 3 discloses surface roughness of the ceramic substrate being less than 0.5 μm.
However, in the case of bonding a metal plate on a ceramic substrate, there is an inconvenience in that a sufficiently high level of bonding strength cannot be obtained if surface roughness of the ceramic substrate is simply reduced, and the level of reliability cannot be improved. For example, even when a dry honing process was performed on the surface of a ceramic substrate using Al2O3 particles, and the surface roughness was made Ra=0.2 μm, interfacial peeling occurred during the peeling test in some cases. Moreover, even when surface roughness was made Ra=0.1 μm by means of a polishing method, interfacial peeling occurred in some cases.
In particular, recently, while the size and thickness of power modules are being reduced, the usage environment thereof is becoming more severe. Moreover, the amount of heat generation from electronic components to be mounted such as semiconductor element tends to increase, and there is a need for arranging a power module substrate over a heat sink. In this case, the power module substrate is restrained by the heat sink, and therefore, a high level of shearing force acts on the bonding interface between the metal plate and the ceramic substrate when a thermal cycle load is being applied. For this reason, more than ever before, there is a demand for increased bonding strength between a ceramic substrate and a metal plate, and for increased reliability thereof.